The present invention relates to a process for producing doped III-N bulk crystals from a gas/vapor phase, as well as to a process for producing free-standing doped III-N substrates which are obtained from the doped III-N bulk crystals. Here, the term N denotes nitrogen and III denotes at least one element of group III of the periodic system, selected from aluminum, gallium and indium (in the following sometimes abbreviated by (Al,Ga,In)). The invention further relates to doped III-N bulk crystals and free-standing doped III-N substrates obtained by such process. These free-standing doped III-N substrates are well-suited for the manufacture of optic and optoelectronic devices.
Conventionally in the commercial use, devices for (Al,Ga,In) N-based light emitting or laser diodes essentially had been grown on foreign substrates such as Al2O3 (sapphire) or SiC.
The disadvantages caused by the use of the foreign substrates with respect to crystal quality and correspondingly lifetime and efficiency of the devices may be coped with only by growing on free-standing III-N-, such as e.g. (Al,Ga)N-substrates. The latter, however, are hardly available in sufficient quality up to now. The reason for this essentially resides in difficulties of conventional bulk growing technologies owing to the extremely high steady-state vapor pressure of nitrogen above III-N compounds at typical growth temperatures.
The growth of bulk material under high pressure had been described by Porowski (MRS Internet J. Nitride Semicond. Res 4S1, 1999, G1.3). This process leads to a qualitatively valuable GaN bulk material, but has the disadvantage that, up to now, only small GaN substrates having an area of maximally 100 mm can be produced. In addition, the manufacturing process, in comparison with other processes, requires a lot of time and, due to the extremely high growth pressures, is technologically laborious.
Another method resides in the growth of III-N material on a foreign substrate from the gaseous/vapor phase with a subsequent separation from the foreign substrate.
For producing thick, free-standing layers of III-N, such as GaN, it is e.g. known from the publication “Large Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy and Laser-Induced Liftoff” by Michael Kelly et al. (Jpn. J. Appl. Phys. Vol. 38, 1999, pp. L217-L219) to separate, from a sapphire substrate, a thick GaN layer previously grown on the sapphire (Al2O3) substrate by means of hydride vapor phase epitaxy (HVPE). In this connection, it is described to irradiate the GaN-coated sapphire substrate by means of a laser, whereby the GaN layer is thermally decomposed locally at the interface to the sapphire substrate and thereby released from the sapphire substrate. Alternative releasing methods consist of wet chemical etching (for example of GaAs; K. Motoki et al., Jap. J. Appl. Phys. Vol. 40, 2001, pp. L140-L143, dry-chemical etching (for example of SiC; Yu. Melnik et al., Mat. Res. Soc. Symp. Proc. Vol. 482, 1998, pp. 269-274), or mechanical lapping (for example of sapphire; H.-M. Kim et al., Mat. Res. Soc. Symp. Proc. Vol. 639, 2001, pp. G6.51.1-G6.51.6) of the substrate.
The disadvantage of the described methods resides, on the one hand, in the relatively high costs owing to laborious technologies for the substrate release, and on the other hand in the basic difficulty to produce III-N material having a homogeneously low defect density.
The growth of thick III-N bulk crystals (boules) on III-N substrate by means of vapor phase epitaxy with subsequent individualization of the bulk crystal by a sawing process offers an alternative to the aforementioned processes.
Such a process has been described by Vaudo et al. (U.S. Pat. No. 6,596,079). As a preferred growing method the HVPE was chosen; as preferred boule crystal length, values of >1 mm, 4 mm or 10 mm had been mentioned. Vaudo et al. further described, inter alia, how to obtain III-N substrates from the bulk crystal by means of wire sawing or other treatment steps, for example chemical-mechanical polishing, reactive ion beam etching or photo-electrochemical etching. In an international patent application of Vaudo et al. (WO 01/68955 A1), III-N bulk crystals and substrates produced by means of the described technology are further mentioned.
Melnik et al. describe a process for growing GaN- (U.S. Pat. No. 6,616,757) or AlGaN-bulk crystals (US 2005 0212001 A1) having crystal lengths greater than 1 cm. There, the process consists of the basic steps: growth of a single-crystalline (Al)GaN layer on a substrate, removal of the substrate, and growth of the (Al)GaN bulk crystal on the single-crystalline (Al)GaN layer. As the preferred method, a HVPE process with a specific reactor structure is mentioned. In addition, Melnik et al. describe, in a US application (US 2005 0164044 A1) or in U.S. Pat. No. 6,936,357, GaN or AlGaN bulk crystals having various properties, such as, for example, sizes, dislocation densities or full widths at half maximum (half widths) of X-ray diffraction curves.
Besides the crystallinity, the electrical properties of semiconductor crystals must also be adapted to the needs of the respective uses. The properties of semiconductor crystals, in particular the electrical properties, can be controlled by the incorporation of foreign atoms, so-called dopants. By the concentration of dopants in a crystal, the concentration of charge carriers and thus the specific electric resistivity can be controlled. For opto-electronic devices, conducting substrates are used in order to allow a contact of the devices through the back-side of the substrate. In the case of GaN or AlGaN substrates, typically an n-doping is chosen, i.e., the incorporation of foreign atoms which generate mobile electrons. For example, a usual dopant for (Al)GaN is silicon. A p-doping is also possible, i.e., the incorporation of foreign atoms which generate holes, i.e., defect electrons. For example, a usual dopant for (Al)GaN is magnesium. Another possibility is represented by the incorporation of foreign atoms which act as low-energy defect sites and thus bond mobile charge carriers and thereby reduce the conductivity of the crystal. For (Al)GaN, this is, for example, possible by iron.
In vapor phase epitaxy, the dopants are typically provided in the form of gaseous compounds. For example, silane, SiH4, can be used for silicon, bis(cyclopentadienyl)magnesium, Mg(C5H5)2, for magnesium, and bis(cyclopentadienyl)iron, Fe(C5H5)2, for iron.
For example, Manabe et al. (U.S. Pat. No. 6,472,690) describe the n-doping of GaN by feeding a silicon-containing gas. Usikov et al. (Mat. Res. Soc. Proc. Vol. 743, L3.41.1) describe the n-doping by feeding silane with the HVPE. There is no mention about the homogeneity.
Vaudo et al. (US publication 2005/0009310 A1) describe semi-insulating GaN crystals having a doping with low acceptors. In the description, metal-organic compounds are mentioned as dopants.
For HVPE growth, chlorine-containing compounds such as dichlorosilane, SiCl2H2, may also be used. Usui et al. (JP 3279528 B) describe doping with SiHxCl(4-x).
The generation of the chloride compound of the dopants in the HVPE process may be carried out, analogous to the generation of GaCl, in situ by the reaction of the elemental dopant with HCl. Thus, the gaseous doping substances may be generated in the reactor via additional gas pipelines in connection with corresponding additional crucibles with the respective elemental starting materials. This process is, for example, described by Fomin et al. (phys. stat. sol. (a) Vol. 188, pp. 433). Hong et al. (U.S. Pat. No. 6,177,292) mention this procedure, in order to produce an n-doped GaN layer on a polished GaN substrate. Nikolaev et al. (U.S. Pat. No. 6,555,452; corresponding to US 2002/28565 A) describe the p-doping by the inclusion of a metallic dopant such as Mg or Zn into an additional source region separated from the III-starting material. This process requires a complex enlargement of gas feedings and sources within the reactor.